15
value is equivalent to -62.5 PSI and should be set to this value, and the HI SCALE is set to
"250". The engineering units are set to "PSI". The display will now show zero to 250 PSI for 1
to 5 Volts input. The linear scaling is shown in Figure 2.5 on the previous page.
2.5 ALARMS
The recorder has four programmable internal alarms with dual relay output. The alarms can
be associated with either analog input channel, can be set as high or low, latching or non-
latching with or without lockout capability. Each relay has a set of potential free normally open
contacts (Form A - SPNO) that are accessible from the rear panel via screw terminal
connections. Alternatively the relays can be set to the failsafe mode, normally closed (Form B
– SPNC). The relays can be assigned to any, all or none of the alarms. Refer to section 4.2
and 4.3.
2.5.1 Alarm Types
A
high
alarm is active when the input is greater than the setpoint. A
low
alarm is active
when the input is less than the setpoint. A
latching
alarm will remain set once activated
and must be
reset
by the operator. Note that the alarm condition should be removed
before resetting the alarm or the alarm will activate and latch again. A
non-latching
limit
will set on alarm condition and automatically reset once the alarm condition is removed. A
differential, known as hysteresis or
deadband
can be introduced so that once an alarm is
set (at the setpoint), the input must go beyond the deadband before the alarm resets.
This prevents the output relays from chattering in borderline conditions. The
deadband
is
set in absolute units and adds to the setpoint in a low alarm, or subtracts from the
setpoint in a high alarm. By way of example, if the setpoint is 100, the hysteresis
(deadband) is set to 5, and the alarm is a high type and non latching, then the alarm will
activate when the input exceeds 100 and will remain active until the input drops below 95
(100 minus 5). Alternatively the user can set a
reset delay
from 1 to 255 seconds. This
delay must time out once an alarm condition has been removed before the alarm will
reset.
Lockout
prevents an alarm from activating until the setpoint has been traversed in the
opposite sense by the input. For example, if we were measuring pressure and had a low
alarm set at 100, we may not wish the alarm to be active on start up when the pressure
may be low or zero. We really wish to monitor a low pressure situation once our target
pressure has been reached, say 150. The lockout prevents the alarm activating at
startup. The alarm remains deactivated until the input exceeds the setpoint. At this point
the alarm arms and any time from this point on that the input drops below the setpoint,
the alarm will activate as normal.
2.5.2 Setting Alarms
Setting and configuration is done in the
Advanced Setup Menu
.
The alarm setpoints are set in the same units as the inputs and may be different for each
channel. When setting the actual setpoint value, the value may be incremented in steps
other than what is expected (unit steps). This is due to the fact that the input is converted
into a digital value with a finite resolution. It is not possible to set a setpoint value that
cannot be resolved by the internal microprocessor. Select the closest value to the exact
value you require; it will always be with 0.5%. The same applies when setting the
deadband.
Note that when configuring the alarms, there is no correlation between inputs, alarms and
relay outputs. These are user defined. Each alarm can be assigned to only one input
channel.
When setting up each alarm channel, the user should DISABLE the alarm so that
parameters may be changed WITHOUT AFFECTING THE RELAYS. This is done in the
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